Immature T cell lymphomas comprise a significant portion of human lymphoid malignancies. Many of these tumors harbor recurrent chromosomal translocations and related aberrations that either activate proto-oncogenes, inactivate tumor suppressor genes, or create novel oncogenic fusion genes. Most oncogenic translocations of human immature T cell lymphomas are thought to occur via errors in the repair of DNA double strand breaks (DSBs) introduced at T cell receptor (TCR) loci during V(D)J recombination and/or general DSBs at other genomic locations. We propose to elucidate functions of the DSB response in suppression of translocations associated with T cell lymphomas and to generate novel mouse models for human T cell lymphoma. We also propose to elucidate molecular mechanisms that underlie recurrent translocations in T cell lymphomas, including how spatial proximity, DSB frequency and DNA repair pathway availability affect translocation patterns. Recurrent chromosome 14 translocations in the vicinity of the TCRa/d locus are found frequently in ATM-deficient mouse thymic lymphomas and similar translocations are found in human T cell lymphomas that have mutated ATM genes. In this regard, we find that a region 10 Mb upstream of the TCRa/d locus is highly amplified on chromosome 14 in most ATM-deficient mouse thymic lymphomas. We propose to fully investigate this recurrent translocation/amplification in ATM-deficient T cell lymphomas i) to elucidate mechanistic aspects, including potential roles of TCRa/d locus V(D)J recombination and TCRa/d enhancers (with Project 2, Harald von Boehmer), ii) to identify target oncogene(s) (with Project 5, Rick Young), and iii) to determine relevance to human T cell lymphomas (with Project 1 Tom Look). For translocations, participating loci on different chromosomes must be broken and must be in close proximity for joining. Thus we propose to test the hypothesis that frequent activation of certain proto-oncogenes via translocation to TCR loci in human, but not mouse, T cell lymphomas may reflect the relative frequency of DNA DSBs and the spatial proximity of target loci. To address this question, we will employ various approaches including 3D FISH and the generation of novel cell culture and mouse models in which DNA breaks are introduced into target T-cell oncogenes during T-cell development. We will also employ these models to test our hypothesis that ATM and its substrates (e.g., H2AX) prevent translocations resulting from aberrant V(D)J recombination by stabilizing TCR locus DSBs introduced during V(D)J recombination. Together, these studies should allow us to address long-standing questions regarding the mechanisms underlying chromosomal translocation targeting in T-ALL and other cancers.